As population demographics in the US continue to skew towards older ages, determining the processes that underlie the physical deterioration that occurs in normal aging becomes of increasing importance. An analysis of the genetics of human aging is possible and promises to yield new insight into these processes. The genes responsible for most genetic disorders of human premature aging have been identified. Werner's syndrome (WS) is one such disorder. The overall aim of this proposal is to use the molecular analysis of WS to better understand the aging process in the normal human population. WS is a human genetic disorder with features suggestive of accelerated aging. The gene defective in most cases of WS encodes a polypeptide with both helicase and exonuclease activities. Multiple studies support a role or roles of the WS protein in telomere maintenance. Telomere loss is predicted to reflect the sum of DNA lost uniformly at all telomeres during replication, and sporadic loss events occurring at individual telomeres with variable frequency. The latter class of events includes loss of telomere sequences due to damage to telomere DNA. Using a simple inhibition of colony formation assay, we have found that primary WS fibroblasts are defective in repair of oxidative telomere damage. Based on our preliminary data and published analyses of the effect of oxidative stress on telomere sequence loss in normal fibroblasts and studies of the role of WS protein in telomere processing, we suggest a model in which the WS protein participates in repair of damage to telomeres. In Aim 1, we will study the nature of the lesions produced by oxidative damage at telomeres and the mechanisms by which the cell recognizes and responds to this damage. In Aim 2, we will study the role of the WS protein in the response to telomere damage caused by oxidative stress. The mechanism by which telomerase suppresses the sensitivity of WS fibroblasts to oxidative stress will be determined in Aim 3. The studies in Aims 1-3 will be performed in tissue culture;to determine the in vivo relevance of the role of oxidative stress in the development of phenotypes related to telomere deficiency, in Aim 4 we will study mice that are deficient in combinations of the WS helicase, telomerase RNA component, and superoxide dismutase 2. These studies provide a novel link between oxidative stress, telomere dysfunction and a human progeroid syndrome. The relative contribution of sporadic loss events and end-replication inefficiency to rates of overall telomere sequence loss is currently unknown. Increased understanding of sporadic loss events and the pathways influencing their occurrence is of critical importance in devising strategies to minimize telomere loss.